2018-11-26

linux_kernel-list.h

看linux的代码看到这段利用头文件实现的链表。这段写的确实经典，简洁（但不算易懂）且用到了C语言的一些奇技淫巧（误），跟数据结构书上的比起来不知道要高到哪去（不过其实我也没有很注意看书上的代码就是了）

写了下注释

/*-
 * Copyright (c) 2011 Felix Fietkau <nbd@openwrt.org>
 * Copyright (c) 2010 Isilon Systems, Inc.
 * Copyright (c) 2010 iX Systems, Inc.
 * Copyright (c) 2010 Panasas, Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice unmodified, this list of conditions, and the following
 *    disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
#ifndef _LINUX_LIST_H_
#define _LINUX_LIST_H_

#include <stddef.h>
#include <stdbool.h>

#define	prefetch(x)

    /*很有趣的一个实现
    __typeof__(((type *) NULL)->member) *__mptr 把mptr初始化为任意结构体的member元素对应类型的指针
    ptr为结构体内某元素的指针
    offsetof(type, member) 返回type结构体的member成员相对type的偏移量
    因此该宏定义作用是传入一个结构体实例的成员指针，返回该结构体的头指针
    */
#ifndef container_of
#define container_of(ptr, type, member)					\
	({								\
		const __typeof__(((type *) NULL)->member) *__mptr = (ptr);	\
		(type *) ((char *) __mptr - offsetof(type, member));	\
	})
#endif

struct list_head {
	struct list_head *next;
	struct list_head *prev;
};

#define LIST_HEAD_INIT(name) { &(name), &(name) }	
           //这里用name的地址初始化prev和next，使name声明后本身是个回环
#undef LIST_HEAD
#define LIST_HEAD(name)	struct list_head name = LIST_HEAD_INIT(name)

static inline void
INIT_LIST_HEAD(struct list_head *list)
{
	list->next = list->prev = list;
}

static inline bool
list_empty(const struct list_head *head)
{
	return (head->next == head);
}

static inline bool
list_is_first(const struct list_head *list,
	      const struct list_head *head)
{
	return list->prev == head;			//如果前一个元素就是链表头
}

static inline bool
list_is_last(const struct list_head *list,
	     const struct list_head *head)
{
	return list->next == head;			//如果后一个元素是链表尾
}

static inline void
_list_del(struct list_head *entry)
{
	entry->next->prev = entry->prev;
	entry->prev->next = entry->next;
}

static inline void
list_del(struct list_head *entry)
{
	_list_del(entry);
	entry->next = entry->prev = NULL;
}					
		//这里没有free操作，说明这些函数不处理内存分配问题

static inline void
_list_add(struct list_head *_new, struct list_head *prev,
    struct list_head *next)
{

	next->prev = _new;
	_new->next = next;
	_new->prev = prev;
	prev->next = _new;
}						//同样没有malloc

static inline void
list_del_init(struct list_head *entry)
{
	_list_del(entry);
	INIT_LIST_HEAD(entry);
}
		/*假设list_head是一个容器（结构体）内的成员
		通过list_entry获取该容器对应实例的头指针
		ptr为指向list_head的指针*/
#define	list_entry(ptr, type, field)	container_of(ptr, type, field)
#define	list_first_entry(ptr, type, field)	list_entry((ptr)->next, type, field)
#define	list_last_entry(ptr, type, field)	list_entry((ptr)->prev, type, field)

#define	list_for_each(p, head)						\
	for (p = (head)->next; p != (head); p = p->next)

#define	list_for_each_safe(p, n, head)					\
	for (p = (head)->next, n = p->next; p != (head); p = n, n = p->next)

#define list_for_each_entry(p, h, field)				\
	for (p = list_first_entry(h, __typeof__(*p), field); &p->field != (h); \
	    p = list_entry(p->field.next, __typeof__(*p), field))
										//h为链表头

#define list_for_each_entry_safe(p, n, h, field)			\
	for (p = list_first_entry(h, __typeof__(*p), field),		\
	    n = list_entry(p->field.next, __typeof__(*p), field); &p->field != (h);\
	    p = n, n = list_entry(n->field.next, __typeof__(*n), field))

#define	list_for_each_entry_reverse(p, h, field)			\
	for (p = list_last_entry(h, __typeof__(*p), field); &p->field != (h); \
	    p = list_entry(p->field.prev, __typeof__(*p), field))

#define	list_for_each_prev(p, h) for (p = (h)->prev; p != (h); p = p->prev)
#define	list_for_each_prev_safe(p, n, h) for (p = (h)->prev, n = p->prev; p != (h); p = n, n = p->prev)

static inline void
list_add(struct list_head *_new, struct list_head *head)
{
	_list_add(_new, head, head->next);
            //这里的第三个参数结合list_init构造了一个循环链表
}

static inline void
list_add_tail(struct list_head *_new, struct list_head *head)
{
	_list_add(_new, head->prev, head);
}

static inline void
list_move(struct list_head *list, struct list_head *head)
{
	_list_del(list);
	list_add(list, head);
}

static inline void
list_move_tail(struct list_head *entry, struct list_head *head)
{
	_list_del(entry);
	list_add_tail(entry, head);
}

static inline void
_list_splice(const struct list_head *list, struct list_head *prev,
    struct list_head *next)
{
	struct list_head *first;
	struct list_head *last;

	if (list_empty(list))
		return;

	first = list->next;
	last = list->prev;
	first->prev = prev;
	prev->next = first;
	last->next = next;
	next->prev = last;
}	
		//将头为prev尾为next（其实头尾一样）的链表拼接到list链表内，注意拼接后list不再是链表头

static inline void
list_splice(const struct list_head *list, struct list_head *head)
{
	_list_splice(list, head, head->next);
}
										//list为待插入链表
static inline void
list_splice_tail(struct list_head *list, struct list_head *head)
{
	_list_splice(list, head->prev, head);
}

static inline void
list_splice_init(struct list_head *list, struct list_head *head)
{
	_list_splice(list, head, head->next);
	INIT_LIST_HEAD(list);
}

static inline void
list_splice_tail_init(struct list_head *list, struct list_head *head)
{
	_list_splice(list, head->prev, head);
	INIT_LIST_HEAD(list);
}
					//插入list后将list头指针初始化
#endif /* _LINUX_LIST_H_ */

函数功能总结

结构体

list_head 双向循环链表

宏

container_of( 指针,成员类型,成员名 ) 传入结构体某个成员的指针返回结构体头指针

函数

INIT_LIST_HEAD 初始化链表

list_empty 判断列表是否为空

list_is_first 判断当前元素是否为第一个

list_is_last 判断当前元素是否为最后一个

===================================

_list_del 删除元素通用实现

list_del 删除head指向元素

_list_add 添加元素通用实现

list_add 头插法，在链表头添加一个元素

list_add_tail 尾插法，链表尾添加一个元素

list_del_init 删除并初始化

list_move 移动list元素到链表头

list_move_tail 移动list到链表尾

===================================

list_entry 获取list_head所在容器的头指针

list_first_entry 上一个容器的头指针

list_last_entry 下一个容器的头指针

===================================

list_for_each

list_for_each_safe 向后遍历链表

list_for_each_prev

list_for_each_prev_safe 向前遍历链表

list_for_each_entry

list_for_each_entry_safe 遍历容器中的链表，p指向容器头

list_for_each_entry_reverse 反向遍历

===================================

_list_splice 链表拼接通用实现

list_splice 头插法将list插入head链表

list_splice_tail 尾插法

list_splice_init

list_splice_tail_init 插入后初始化

===================================

一个对于CONTAINER_OF和LIST_ENTRY解释得不错的链接

https://blog.csdn.net/hs794502825/article/details/10364977

几个比较有意思的宏

offsetof

出处：openwrt/libubox/list.h

1	#define offsetof(s,m) (size_t)&(((s *)0)->m)

这个宏的作用是求结构体的一个成员相对结构体首地址的偏移量

1	((s *)0)->m

跟

1	((s *)NULL)->m

等价，这里做了件事，就是将数字0（NULL）强制转换成一个地址，使它的类型为指向结构体的指针s*
因此如果调用 offsetof(A,pointer)实际上调用了

1	(size_t) &((A *)NULL)->pointer

假设pointer成员相对A类型头指针的偏移量为n，那么上述运算的结果即为n
因为 &((A)NULL) 值为0，所以 &(((A )NULL)->pointer)值为n
这里注意符号&，原因我就不多说了

container_of

出处：openwrt/libubox/list.h

#define container_of(ptr, type, member)					\
({								\
	const __typeof__(((type *) NULL)->member) *__mptr = (ptr);	\
	(type *) ((char *) __mptr - offsetof(type, member));	\
})

这个宏用处在于假设定义了一个结构体

typedef struct mystruct{
    int num;
    list_head pointer;
} A;

这里list_head是双向链表的指针，定义如下

typedef struct _list_head{
	struct *list_head prev;
	struct *list_head next;
} list_head;

定义

1 2	A a; int p = (int) &a.pointer.next; //这里我强制转换成(int*)是省事而已，其实是个不好的做法

此时调用

1 2	void *p = NULL; p = container_of(p,A,pointer);

会得到指向a的指针，原因如下，把宏展开

1 2	p = ({ const __typeof__( ( (A )NULL ) ->pointer) mptr = (p); (A) ( (char) __mptr - offsetof(A,pointer) ) })

还是有点难懂，拆分一下
第一个问题是，mptr的类型究竟是什么，答案是

1	const __typeof__( ( (A *)NULL ) ->pointer)

再拆分，typeof( ( (A )NULL ) ->pointer)是什么？，其实就是A类型的pointer这个成员的类型，即list_head
typeof是gnu C编译器的一个宏，返回变量的类型
所以mptr的类型是 const list_head
所以这个宏的第一步是

1	const list_head * mptr = (p);

再看第二步

1	(A) ( (char) __mptr - offsetof(A,pointer) )

第一步定义的mptr减去mptr指向的成员（在本例中是pointer）相对A结构体首地址的偏移
因此得到的就是该A结构体实例的首地址，即a，这也是为什么第二步的最外层是个(A*)的强制转换